Full-Scale Fracture Toughness Behavior of Zr-2.5Nb Pressure Tubes with High Hydrogen Concentrations and Different Hydride Morphologies

2021 ◽  
pp. 262-293
Author(s):  
Jun Cui ◽  
Gordon K. Shek
Keyword(s):  
Fracture Toughness ◽  
Full Scale ◽  
High Hydrogen ◽  
Pressure Tubes

Fracture Protection of CANDU Zr-2.5Nb Pressure Tubes With High Hydrogen Equivalent Concentration

2015 ◽  
Author(s):  
Preeti Doddihal ◽  
Douglas Scarth ◽  
Paula Mosbrucker ◽  
Steven Xu
Keyword(s):  
Fracture Toughness ◽  
Heavy Water ◽  
Pressure Tube ◽  
Normal Operation ◽  
Tube Material ◽  
High Hydrogen ◽  
Equivalent Concentration ◽  
Corrosion Reaction ◽  
Pressure Tubes ◽  
Heavy Water Reactor

The core of a CANDU®1 (CANada Deuterium Uranium) pressurized heavy water reactor includes horizontal Zr-2.5Nb alloy pressure tubes that contain the fuel. Pressure-temperature limits are used in CANDU® reactors for normal operation heat-up and cool-down conditions to maintain margins against fracture. The pressure-temperature limits are determined by postulating a 20 mm long axial through-wall crack in the pressure tube and using a fracture toughness-based calculation procedure. Due to a corrosion reaction with the heavy water coolant, pressure tubes absorb deuterium isotope in service, resulting in an increase in hydrogen equivalent concentration. Experiments have shown that high hydrogen equivalent concentration reduces the fracture toughness of pressure tube material at low temperatures during reactor heat-up and cool-down from normal operating temperatures. New fracture toughness curves that are applicable to material with high hydrogen equivalent concentration have been developed to address this issue. These curves are being used to develop new pressure-temperature limits for fracture protection of CANDU® pressure tubes. The methodology for deriving the pressure-temperature limits for a CANDU® Zr-2.5Nb pressure tube using the new fracture toughness curves is presented in this paper. Preliminary results of pressure-temperature limits for a CANDU® reactor are also provided.

Quantitative Characterization of Hydride Spacing for Cohesive-Zone Modelling of Fracture Toughness in Zr-2.5Nb Pressure Tubes

2016 ◽  
Author(s):  
Cheng Liu ◽  
Leonid Gutkin ◽  
Douglas Scarth
Keyword(s):  
Fracture Toughness ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Model Development ◽  
Threshold Value ◽  
Pressure Tube ◽  
Zone Model ◽  
Empirical Distributions ◽  
Hydride Formation ◽  
Pressure Tubes

Zr-2.5Nb pressure tubes in CANDU 1 reactors are susceptible to hydride formation when the solubility of hydrogen in the pressure tube material is exceeded. As temperature decreases, the propensity to hydride formation increases due to the decreasing solubility of hydrogen in the Zr-2.5Nb matrix. Experiments have shown that the presence of hydrides is associated with reduction in the fracture toughness of Zr-2.5Nb pressure tubes below normal operating temperatures. Cohesive-zone approach has recently been used to address this effect. Using this approach, the reduction in fracture toughness due to hydrides was modeled by a decrease in the cohesive-zone restraining stress caused by the hydride fracture and subsequent failure of matrix ligaments between the fractured hydrides. As part of the cohesive-zone model development, the ligament thickness, as represented by the radial spacing between adjacent fractured circumferential hydrides, was characterized quantitatively. Optical micrographs were prepared from post-tested fracture toughness specimens, and quantitative metallography was performed to characterize the hydride morphology in the radial-circumferential plane of the pressure tube. In the material with a relatively low fraction of radial hydrides, further analysis was performed to characterize the radial spacing between adjacent fractured circumferential hydrides. The discrete empirical distributions were established and parameterized using continuous probability density functions. The resultant parametric distributions of radial hydride spacing were then used to infer the proportion of matrix ligaments, whose thickness would not exceed the threshold value for low-energy failure. This paper describes the methodology used in this assessment and discusses its results.

Prediction of Failure Behavior for Nuclear Piping Using Curved Wide-Plate Test

2004 ◽  
Vol 126 (4) ◽  
pp. 419-425 ◽  
Author(s):  
Nam-Su Huh ◽  
Yun-Jae Kim ◽  
Jae-Boong Choi ◽  
Young-Jin Kim ◽  
Chang-Ryul Pyo
Keyword(s):  
Fracture Toughness ◽  
Three Dimensional ◽  
Crack Tip ◽  
Full Scale ◽  
Failure Behavior ◽  
Resistance Curve ◽  
Testing Specimen ◽  
Plate Test ◽  
Crack Tip Constraint ◽  
Three Dimensional Finite Element

One important element of the Leak-Before-Break analysis of nuclear piping is how to determine relevant fracture toughness (or the J-resistance curve) for nonlinear fracture mechanics analysis. The practice to use fracture toughness from a standard C(T) specimen is known to often give conservative estimates of toughness. To improve the accuracy of predicting piping failure, this paper proposes a new method to determine fracture toughness using a nonstandard testing specimen, curved wide-plate in tension. To show validity of the proposed curved wide-plate test, the J-resistance curve from the full-scale pipe test is compared with that from the curved wide-plate test and that from C(T) specimen. It is shown that the J-resistance curve from the curved wide-plate tension test is similar to, but that from the C(T) specimen is lower than, the J-resistance curve from the full-scale pipe test. Further validation is performed by investigating crack-tip constraint conditions via detailed three-dimensional finite element analyses, which shows that the crack-tip constraint condition in the curved wide-plate tension specimen is indeed similar to that in the full-scale pipe under bending.

Flaw Evaluation Procedure for Cast Austenitic Stainless Steel Materials Using Thermal Aging Models

2017 ◽  
Author(s):  
M. F. Uddin ◽  
G. M. Wilkowski ◽  
S. Pothana ◽  
F. W. Brust
Keyword(s):  
Fracture Toughness ◽  
Thermal Aging ◽  
Austenitic Stainless Steels ◽  
Full Scale ◽  
Evaluation Procedure ◽  
Chemical Compositions ◽  
Large Variability ◽  
Acceptance Criterion ◽  
Operating Temperatures ◽  
Aging Models

Thermal embrittlement of cast austenitic stainless steels (CASS) can occur at reactor operating temperatures potentially leading to a reduction in their fracture toughness. Some aged CASS materials have the potential to have exceedingly low toughness and also show high toughness variability due to the nature of their microstructure. The experimentally measured JIc values for CASS materials showed a large scatter when plotted against ferrite number (FN) or chrome equivalent number (Creq). Because of their low aged toughness with such a large variability, flaw evaluations of CASS material needs to be done carefully, especially since most US PWR nuclear plants have been given plant-life extensions for 60-year operation, and consideration of further extension to 80 years is underway. However, the ASME Section XI Appendix C flaw acceptance criterion currently does not have a recommended procedure for flaw evaluation for CASS materials with FN ≥ 20, and the Working Group recognizes that the changes might also be needed for CASS with FN less than 20. In this paper, a flaw evaluation procedure for fully aged CASS materials is presented using JIc values at LWR operating temperatures predicted from several existing thermal-aging toughness degradation models. All available thermal aging models for CASS materials were evaluated which predict fully aged (lower saturated toughness condition) fracture toughness of CASS based on their chemical compositions. A set of 20 experimental test data was analyzed by using all models to find the most accurate thermal aging models. Using the most accurate models, correlations between predicted JIc values and French Creq-Fr and ASTM A800 FN were developed from a database of 274 pipe/elbows in US PWR plants whose chemical compositions were known. Finally, the correlation was used to determine the elastic-plastic fracture correction factor (Z factor) for CASS pipe and fittings as a function of pipe diameter and their chemical compositions from material certification sheet using the Dimensionless-Plastic-Zone-Parameter (DPZP) analysis. The DPZP analysis is a relatively simple curve-fitting procedure through full-scale circumferential surface-cracked pipe tests developed in pipe fracture projects funded by the USNRC, and was checked against a full-scale aged CF8m pipe fracture test. After determining the chemical composition specific Z factor for CASS materials, the flaw evaluation can be performed according to the ASME Section XI Appendix C procedures.

Tensile properties and fracture toughness of Zr–2.5Nb alloy pressure tubes of IPHWR220

2015 ◽  
Vol 293 ◽  
pp. 138-149 ◽  
Author(s):  
H.K. Khandelwal ◽  
R.N. Singh ◽  
A.K. Bind ◽  
S. Sunil ◽  
J.K. Chakravartty ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Tensile Properties ◽  
Pressure Tubes

scholarly journals Considerations in selecting laboratory scale test specimens for evaluation of fracture toughness

2013 ◽  
Vol 4 (1) ◽  
Author(s):  
Matthias Verstraete ◽  
Stijn Hertelé ◽  
Koen Van Minnebruggen ◽  
Rudi Denys ◽  
Wim De Waele
Keyword(s):  
Fracture Toughness ◽  
Finite Element ◽  
Test Specimen ◽  
Tensile Specimen ◽  
Full Scale ◽  
Laboratory Scale ◽  
Single Edge ◽  
Apparent Fracture Toughness ◽  
Edge Notch ◽  
Scale Test

  The assessment of defects in large steel structures requires a trustworthy evaluation of the material’s toughness. This toughness is not only a material property but is also influenced by the loading conditions and geometry; the so-called constraint. The resulting representative value is referred to as the apparent toughness. The evaluation of apparent fracture toughness in a flawed structure is preferentially performed through laboratory scale testing, as full scale tests are both expensive and often challenging to perform. Several laboratory scale test specimens are available, among which a Single Edge Notch Bending specimen, Single Edge Notch Tensile specimen, Double Edge Notch Tensile specimen and Centre Cracked Tensile specimen. Each of these specimens has its own specific constraint. Therefore, the selection of an appropriate test specimen is of primary importance for limiting the conservatism and avoiding potential unconservatism with respect to full scale behaviour. This paper provides a general framework to select an appropriate test specimen based on detailed finite element simulations of both the full scale structure and the laboratory scale test specimens. These finite element calculations allow for a characterization of the crack tip stress fields in both situations. Different theoretical frameworks are available for this characterization; theQ -parameter is considered in this paper. To demonstrate the applicability of this procedure, an example case is presented for circumferentially oriented defects in pressurized pipelines under longitudinal tension. It is concluded that the presented framework allows for efficiently selecting a laboratory scale test specimen, which enables to evaluate the apparent fracture toughness for a given large scale structure. The obtained toughness can thus be incorporated in analytical flaw assessment procedures, reducing the degree of conservatism. This in turn allows an economically effective design.

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